1. Technical Field
The present invention relates to an encoder that converts amount of mechanical displacement of a rotating object to be measured into an electrical signal, processes the signal, and detects a rotation angle and a rotation speed.
2. Description of Related Arts
Encoders have been known as a device to detect a rotation angle (rotation position) and a rotation speed of a rotating object to be measured such as a servo motor.
A typical encoder includes a rotating disk connected to a rotation axis of a rotating object to be measured, and a light-emitting device and a light-receiving device that sandwich the rotating disk to face each other. An optical pattern having a light-transmitting portion and a light-shielding portion is formed on the rotating disk according to a predetermined rule.
Light emitted from the light-emitting device is shielded by the light-shielding portion of the optical pattern on the rotating disk, or is transmitted through the light-transmitting portion and is received by the light-receiving device. Timing of receiving light by the light-receiving device depends on the optical pattern. Therefore, a rotation angle of the rotating disk can be detected by measuring the light-receiving timing as an electrical signal.
A plurality of tracks is configured such that the optical pattern appearing in one rotation of the rotating disk has different cycles, so that detection accuracy of the rotation angle can be enhanced. The more the detection accuracy is enhanced, the larger the number of tracks is, and the rotating disk also becomes larger. This cannot meet the need of products that require downsizing, such as a motor.
So, as a technology to reduce the number of tracks and to downsize the rotating disk, an encoder is disclosed, in which a magnet is attached to a rotation axis along with a rotating disk having an optical pattern (For example, see Japanese Patent Application Laid-Open No. 2009-294073). This encoder identifies a rotation angle with certain accuracy by detecting a magnetic pattern of the magnet, and further enhances the detection accuracy of the rotation angle by combining the magnetic pattern with the optical pattern.
By the way, the encoder disclosed in Japanese Patent Application Laid-Open No. 2009-294073 detects both of the optical pattern and the magnetic pattern as analog data of a sine wave or a cosine wave. If all of the detection data are analog data, all of the detection data need to be subjected to analog-digital conversion and signal processing. Therefore, the encoder is required to be provided with a plurality of analog-digital conversion circuits for the optical pattern and the magnetic pattern individually, and this results in an increase of a device cost.